1. Field of the Invention
The present invention relates to a signal processing apparatus and a signal processing method for detecting a digital signal, and relates in particular to a signal processing apparatus and a signal processing method which employs PRML (Partial Response Maximum Likelihood detection) technology for a magnetic recording system.
2. Related Arts
A code sequence reproduction method employing PRML (Partial Response Maximum Likelihood) technology is well known in magnetic recording. PRML technology integrates maximum likelihood detection with a PR channel and Viterbi detecting method which is used for error correction. The PR method is the transmission method for which efficient code transmission is enabled by permitting a wave on the reception side to use a form of intersymbol interference. In accordance with the form of the intersymbol interference that is used, the PR system comprises a plurality of types: PR class 4 with polynomial 1xe2x88x92D2 and EPR (Extended Partial Response) class 4 with polynomial 1+Dxe2x88x92D2xe2x88x92D3. The PR class 4 and its extended class (EPR class 4) are mainly employed for magnetic recording system, such as magnetic disks.
The Viterbi detecting is one of the maximum likelihood detecting used for error correction on the PR channel. With this method, the intersymbol interference applied by the PR channel is employed to correct errors in a signal read from a recording medium. Specifically, as the read signal has the intersymbol interference, correlative patterns are produced for a code sequence obtained by sampling the signal. Therefore, errors can be detected by comparing the correlative patterns with the actual sampling results, and to perform this comparison and find the pattern which most closely resembles the sampling results, the Viterbi detecting method is used.
The PRML technology is described in detail in, for example, xe2x80x9cCorrelative level coding and maximum-likelihood decodingxe2x80x9d, Hisashi Kobayashi, IEEE Transactions in Information Theory, vol. IT-17, NO.5 pp586-594, September 1971, and xe2x80x9cVitervi Detection of Class IV Partial Response on Magnetic Recording Channelxe2x80x9d, Roger W. Wood and David A. Petersen, IEEE Transactions on Communications, vol. COM-34, NO.5 pp454-461, May 1986.
FIG. 8 is a block diagram showing a conventional signal processing apparatus. In FIG. 8, the equalizer 3 receives a readback signal and equalizes (alters the shape of) the received signal to produce a signal having a predetermined PR characteristic (e.g., PR class 4: PR4). Then, an analog-digital (A/D) converter repeatedly samples the PR equalized signal and converts into a quantized signal which is subsequently transmitted to a maximum likelihood detector 10. Available to the maximum likelihood detector 10 are all the branch node combinations provided by the intersymbol interference characterized by PR4 channel. FIG. 8 shows trellis diagram of the Viterbi detector. There are two nodes and four kind of branch transmissions: (1) node M+ to node M+; (2) node M+ to node Mxe2x88x92; (3) node Mxe2x88x92 to node M+; and (4) node M+ to node Mxe2x88x92. The node M+ and the node Mxe2x88x92 represent the opposite magnetization directions of individual bits on the recording medium.
The conventional maximum likelihood detector calculates the Euclid distance between the output of A/D and an expected value of +1, 0 or xe2x88x921 for the PR4, and selects a branch for which the Euclid distance is the smallest. Assuming that a sampling signal level is y, the Euclid distance takes values (yxe2x88x921)2, y2 or (y+1)2. For example, when Euclid distance y2 is the smallest in the current node M+, the next node is also node M+, i.e., the branch (1) above. When Euclid distance (y+1)2 is the smallest, the next node is Mxe2x88x92, i.e., the branch (2). Because of the PR4 characteristics, Euclid distance (yxe2x88x921)2 is never the smallest when the current node is M+. Furthermore, when Euclid distance y2 is the smallest in the current node Mxe2x88x92, the next node is also Mxe2x88x92, i.e., the branch (4). When Euclid distance (yxe2x88x921)2 is the smallest, the next node is M+, i.e., the branch (3). Because of the PR4 characteristics, Euclid distance (y+1)2 is never the smallest when the current node is Mxe2x88x92. When a branch is selected, such as (2), node M+ to node Mxe2x88x92, or (3), node Mxe2x88x92 to node M+, for which magnetic transition occurs, code xe2x80x9c1xe2x80x9d in a code sequence is reproduced. When a branch is selected, for which magnetic transition does not occur, such as (1), node M+ to node M+, or (4), node Mxe2x88x92 to node Mxe2x88x92, code xe2x80x9c0xe2x80x9d in a code sequence is reproduced. The code sequence which is reproduced in this manner is the most likely code sequence.
However, as the recording density is increased, a nonlinear transition shift (NLTS) that a magnetic transition location is affected by an adjacent magnetic transition location and is shifted, and a partial erasure (PE) that two transition carrying opposite magnetic charges locally annihilate each other, tend to occur. That is, in the situation where these nonlinear phenomena occur, the levels which correspond to sequential magnetic transitions at adjacent magnetic transition locations are not always accurately read. Since the Euclid distance does not consider the nonlinear phenomena (NLTS and PE), the code sequence selected based on the Euclid distance by the maximum likelihood detector 10 is not always the most likely code sequence, and performance of the maximum likelihood detector 10 is degraded.
It is, therefore, one objective of the present invention to provide a signal processing apparatus and a signal processing method for precisely detecting a signal read from a magnetic recording medium, even when a nonlinear phenomenon occurs in the channel.
To achieve the above objective, according to the present invention, when the PRML technology is employed to reproduce a code sequence recorded on a magnetic recording medium, the maximum likelihood code sequence of a readback signal is obtained by using a likelihood function based on a conditional distribution of each branch corresponding to a quantization level. A plurality of conditional distributions having various deviations are provided, and a suitable conditional distribution is selected in accordance with the characteristic of the readback signal, the signal reproduction performance can be improved. Especially set is a conditional distribution for the nonlinear phenomenon, such as the nonlinear transition shift (NLTS) and the partial erasure (PE). Therefore, it is possible to prevent the signal processing performance deterioration, caused by the nonlinear phenomenon, which tends to appear more often in consonance with the increase in the recording density of a magnetic recording medium. In addition, the recording density for a magnetic recording medium can be increased.
To achieve the above objective of the present invention, there is provided a signal processing apparatus for converting a signal read from a recording medium by using magnetic recording into a code sequence, comprising:
an equalizer for equalizing the read signal to obtain a signal having a predetermined equalization characteristic;
a quantizing unit for obtaining a quantization level of the equalized signal with each sampling timing; and
a maximum likelihood detector for determining a likelihood code sequence by using a likelihood function based on conditional distributions of each branch corresponding to the obtained quantization level, each of the conditional distributions being defined by a channel equalization characteristic.
With this arrangement, the conditional distribution can be optimized dynamically and the signal reproduction performance can be improved.
It is preferable that a plurality of conditional distributions having various deviations are provided for each of branches that the magnetic transition occurs; and
said maximum likelihood detector selects one of the plurality of conditional distributions based on judging whether or not the magnetic transition occurs in an branch at least immediately before.
As a result, in case that the nonlinear phenomenon is included in the read channel, the deterioration of signal process performance can be prevented.
Further, for example, provided are a plurality of conditional distributions having various deviations corresponding to the characteristic of the readback signal, such as a conditional distribution corresponding to error rate and a conditional distribution according to the length of code xe2x80x9c0xe2x80x9d, in case that the signal conforms to the rule for run length limited (RLL) code. As a result, the signal reproduction performance can be improved.
Furthermore, to achieve the objective of the present invention, there is provided a signal processing method for converting a signal read from a recording medium by using magnetic transition into a code sequence, comprising the steps of:
equalizing the read signal to obtain a signal having a predetermined equalization characteristic;
obtaining a quantization level of the equalized signal with each sampling timing; and
determining a likelihood code sequence by using a likelihood function based on conditional distributions of each branch corresponding to the obtained quantization level, each of the conditional distributions being defined by the channel equalization characteristic.